1. Field of the Invention
The present invention relates to a ceramics joint structure and a method of producing the same.
2. Description of Related Art
Nowadays, electrostatic chucks are being used to adsorb and hold semiconductor wafers in transferring, light-exposing, film-forming such as CVD and sputtering, fine processing, washing, etching, dicing, etc. for semiconductor wafers. Dense ceramics are used as substrates for such electrostatic chucks. Particularly, in the semiconductor producing apparatus, a halogen based corrosive gas such as ClF3, CF4, NF3 is often used as an etching gas or a cleaning gas. In order to rapidly heat and cool the semiconductor wafer while being held by such a substrate, the substrate of the electrostatic chuck desirably has a high heat conductivity. Further, it is desired that the substrate has such thermal shock resistance as not to be destroyed by rapid temperature changes. Dense aluminum nitride and alumina have high corrosion resistance against the halogen based corrosion gas mentioned above.
In the field of the semiconductor producing apparatuses, susceptors in which built-in high frequency electrodes for generating plasma have been practically used. In the field of such high frequency electric power generating apparatuses, a metal electrode is embedded in a substrate of aluminum nitride or dense alumina. Further, in the field of the semi-conductor producing apparatuses, there are used ceramics heaters in which a metal resistor is embedded in a substrate of aluminum nitride or alumina for controlling the temperature of a wafer in each process.
In these apparatuses, it is necessary that the metal electrode is embedded in the ceramic substrate made of aluminum nitride or the like, and that the metal electrode is electrically coupled to an external electric power supply connector. However, a coupling portion for this purpose is exposed to a heat cycle between extreme high temperatures and low temperatures in an oxidative atmosphere and further in a corrosive gas atmosphere. Such a joint portion is desired to maintain high joining strength and excellent electric coupling performance for a long time period even under such a bad condition.
The present inventors have continuously studied the coupling structures as mentioned above. For example, Japanese patent application No. 8-24,835 disclosed that a tip of an electric power supply connector is coupled to a metal electrode in a susceptor with an Al alloy brazing material, a Cu alloy brazing material or a Ni alloy brazing material each having a corrosive resistance. Further, Japanese Patent Laid-Open Publication No. 8-277,173 (JP-A-8-277,173) proposed that a mesh or net metal electrode is embedded in an AIN ceramics, while a part of the mesh or net metal electrode is exposed, and the exposed mesh or net portion and the AIN ceramics are brazed to a tip face of the electric power supply connector. In the techniques mentioned above, there are proposed brazing methods having a high corrosive resistance against the halogen based corrosive gas and its plasma. Moreover, the present inventors further proposed, in Japanese patent application No. 9-12,769, (U.S. Ser. No. 09/013,045), a specific connector-metal electrode joint structure which can maintain high joining strength and excellent conductive performance even if high temperatures or heat cycle is applied thereto under an oxidative atmosphere.
These techniques mentioned above were very effective. However, during the inventors"" further investigation, the following drawbacks were found in an extreme case. That is to say, a molybdenum resistance heater and a molybdenum terminal were embedded in a ceramics heater, and the terminal was brazed to an external electric power supply connector by using an active silver brazing material. In this case, when the ceramics heater was operated for a long time period at, for example, 700xc2x0 C., the molybdenum terminal was corroded. In an extreme case, a joint structure was broken, or a compound having a low melting point was leached on a surface of the heater to cause a bad insulation performance. Further, a silver component was moved to a surface of the heater by its migration under direct or alternating current to cause also a bad insulation performance.
It is an object of the invention to provide a ceramics joint structure and a method of producing the same, in which a corrosion of an embedded member and a bad insulation performance due to a leaching of a compound having low melting point on a surface of a ceramics member or due to a migration of a metal component are prevented even if it is exposed in an oxidative atmosphere such as air during a long term operation at a high temperature region.
According to the invention, a ceramics joint structure in which a ceramics member having an oxidation resistance property and a metal member are joined via a joint layer, comprises a structure such that: (1) an embedded member made of a metal including at least molybdenum is embedded in said ceramics member; (2) a part of said embedded member is exposed to a joint surface of said ceramics member, which is contacted with said joint layer to form a metal exposing portion; (3) said ceramics member and said metal exposing portion are joined via said joint layer to said metal member respectively; and (4) said joint layer is mainly made of one or more metal selected from the group consisting of gold, platinum and palladium.
Moreover, according to the invention, a method of producing the ceramics joint structure mentioned above, comprises the steps of: forming said metal exposing portion by exposing a part of said embedded member to said joint surface of said ceramics member; arranging a material for said joint layer between said joint surface and said metal member; and heating them under an oxidation resistance atmosphere to join said ceramics member and said metal exposing portion to said metal member respectively.
The inventors investigated a cause of the corrosion and the bad insulation performance of the molybdenum terminal. During this investigation, it was found that a minor amount of oxidative atmosphere such as air leaked from a brazed portion reaches to the molybdenum terminal and oxidizes molybdenum to generate molybdenum trioxide, and that the thus generated molybdenum trioxide reacts with silver to generate a compound oxide AgMoO4 having a low melting point. This compound having a low melting point further corroded molybdenum to cause a resistance increase or to, in an extreme case, cause a breakage of a joint portion. In addition, this compound having a low melting point was leached on a surface of a ceramics member to cause a bad insulation performance.
According to the present invention, the inventors contrived the structure in which a part of the embedded member is exposed to a joint surface of the ceramics member, which is contacted with the joint layer to form a metal exposing portion; the ceramics member and the metal exposing portion are joined via the joint layer to the metal member respectively; and the joint layer is mainly made of one or more metals selected from the group consisting of gold, platinum and palladium. In this case, the ceramics member and the embedded member were firmly joined to the external metal member, and thus it was possible to prevent a decrease of joining strength and a decrease of insulation resistance. Moreover, it was found that the migration is not caused under direct or alternating current by making the joint layer from one or more metals selected from the group consisting of gold, platinum and palladium.